I understand during the normal pre-flight testing of the WX radar, that one can inadvertantly transmit with a certain sequence of selections. Can anyone enlighten me to this sequence, just to make sure I avoid it!

If you move the throttle levers past 37 degress it will activate the radar(even if eng's off). I don't know if that is part of the pre-flight, but I could see moving the right throttle forward to test the take off warning horn.

A concern was raised on another (very highly regarded) aviation forum about ATC Transponder selections (and running engines) triggering the Radar-based Predictive Windshear (a serious concern now that ATC selection is now required for taxying at some airports).

I don't know if this is aircraft type-based or Radar system manufacturer-based, but here is a description of one Airbus setup

"On our aircraft, if you select the transponder to ON or AUTO while on the ground, the R/T for the radar will be powered if you also have oil pressure on both engines (even with the main radar system switch in the OFF position). There is a separate switch for the PWS that can be turned from AUTO to OFF to disable the PWS. This will generate a "NAV PRED W/S DET FAULT" ECAM."

1. Predictive Windshear(PWS) turns on when the airplane is on the ground and has one of these conditions;
-THROTTLE/S MOVE THROUGH 53 DEGREES(throttle lever angle)
-FLIGHT CREW PUSHES WXR BUTTON ON AN EFIS CONTROL PANEL The WXR button on the EFIS CP does not turn off the PWS if the throttle/s are above 53 degrees.PWS turns off when the aircraft climbs above 2300feet radio altitude. RADAR WILL RADIATE

Weather Radar Mode .............................Deselect TEST WXR(efis control panel)............................on Weather Radar Mode...............................TEST
verify amber windshear caution,
red windshear warning,
PWS annunications displays momnetarily NOTE;IN THE TIME THE WEATHER RADAR IS ON AND NOT IN THE TEST POSITION IT WILL RADIATE

This means that the engines don't have to necessarily be running to trigger the PWS. Microswitches in the thrust pack send earth/ground signals to the Wx Radar transceiver when the levers are below 53 degrees, and these are lost above 53 degrees.